It is when you form a bubble of spit on your tongue and blow it off with just the right wind speed that it doesn't pop. The bubble then lands on a surface but does not go more then a few seconds before succumbing to a puddle of saliva.

Johnny can now blow spit bubbles on command without any trouble. Because as a child the dumb fuck swore so much that the dishsoap his mom used to rinse his mouth out with permanently fixated itself in his saliva.

Normal Person: Aw fuck man how long have you been blowing those on my food?
Normal Person 2: You know when you complained about somehow getting herpes?
Normal Person: Yea but that was a month ago...
Normal Person 2: ...